In the aircraft and aerospace industries, chemical milling techniques are utilized to selectively etch portions of a metal substrate, such as an aluminum aircraft fuselage panel, in order to form a lightweight structure. In a conventional chemical milling procedure, a maskant is applied to the outer surfaces of the substrate. Conventional maskant formulations are cured by drying to form a chemical-resistant coating. After the maskant composition is cured, a pattern of lines is scribed into the maskant using a laser or a sharp instrument, such as a knife. The scribed lines define “cut-out” portions of the maskant that may be peeled away from the metal substrate in order to expose selected portions of the metal substrate. After a portion of the maskant is removed, the substrate is exposed to an etching solution. Thereafter, additional portions of the maskant may be removed and the etching process repeated.
For process efficiency, it is generally desirable to scribe all of the lines into the maskant film at one time. However, when the etching process comprises multiple stages with certain portions of the maskant being removed at each stage, the presence of the scribed lines can lead to penetration of the etching solution through the maskant in undesirable areas. To avoid this, conventional chemical milling processes include application of a line sealant composition to all of the scribed lines prior to removing portions of the maskant for chemical etching. The line sealant composition protects the metal substrate from chemical exposure in areas where the maskant “cut-out” has not yet been removed.
The water-based or organic solvent-based maskant and line sealant compositions conventionally used to protect the metal substrate during chemical milling processes suffer from a number of disadvantages. For example, conventional line sealant compositions typically last only one to two hours and have a fairly high failure rate, meaning the sealant composition allows the etching solution to penetrate to the metal substrate in undesired locations. In addition, the solvent-based maskant and line sealer compositions are toxic, resulting in increased process cost to address environmental and worker safety issues. Further, the high failure rate of conventional line sealant compositions necessitates the application of multiple line sealant coatings, which also increases process cost and reduces process efficiency. The conventional maskant and line sealer coatings also require drying times that are undesirably long, particularly in high humidity environments. It can take three to four hours or even longer to dry the line sealant and maskant compositions, which further delays the chemical milling process.
There is a need in the art for better methods of protecting metal substrates from chemical exposure during treatments such as chemical milling processes.